This invention relates to a wire scribed circuit board product and method of manufacturing thereof. More specifically this invention relates to a method and device wherein conductive filaments fixed to a substrate are terminated in such a way that an electronic, electro-optical and/or an optical device may be appropriately soldered, bonded, or wire bonded or otherwise connected directly to the conductive filament.
In recent times the electronics industry has moved toward more compact integrated circuit packages with surface-mounted terminals. The density of wiring patterns have substantially increased within the recent past. Many circuit board applications today utilize insulated wires in order to cross the wires over each other, thereby allowing signal paths to run in both X and Y directions on the same layer without the use of via holes.
Although the use of via holes may be avoided, interconnections from wire ends to component pads on the surface are effected by plated through holes. A wire scribed circuit boards can be produced by following the methods described in U.S. Pat. No. 3,674,914. According to previous methods an isolated wire is scribed onto an insulating base by feeding a continuous strand of wire on to the surface of the base while simultaneously affixing the wire to the base thereby producing a wire image of a predetermined interconnect pattern. Holes are subsequently drilled at the terminal points intersecting the termination points of the conductors. The holes are then metallized to electrically connect the termination points of the conductors to surface terminal pads or later added components. As a result the signal path is discontinued when the wire is first connected to the plated metal, and eventually the added component is coupled to the plated metal in the through hole. Furthermore, through holes must be cleaned and chemically etched to provide good adhesion for the copper plating, and to remove the insulation on the exposed wire end in the hole wall.
Another type of wire used for scribing on circuit boards is coaxial wire which is the preferred transmission medium for high frequency signals. Because many of today's circuit boards are designed for very high frequency signals, the signal paths must be treated as transmission paths with the same characteristics such as impedance, cross talk, and propagation delay. Therefore the use of coaxial wires has become appropriate for such high frequency signal applications. A coaxial conductor includes a signal conductor covered by an insulating medium. The signal conductor and the insulating medium are protected by a shield which is electrically connected to the reference potential or ground. A coaxial conductor contains all of the signal energy inside its shield on the signal conductor. The shield isolates the signal conductor from the surrounding environment, thus providing a clean uniform environment for signal travel. The shield prevents radiation of energy from the signal conductor which improves signal propagation and reduces electrical interference which may affect other signals in the vicinity.
The process described above is not applicable if coaxial conductors are used as the interconnect medium. The signal conductor and shield would become electrically connected or "shorted" at the terminal points. An example of a termination method for coaxial wires is disclosed in U.S. Pat. No. 4,908,939 "Method Of Making Coaxial Interconnection Boards" by Shieber, et al and U.S. Pat. No 4,743,710 "Coaxial Interconnection Boards" by Shieber, wherein coaxial conductors are scribed onto a circuit board substrate such that the shorting problem would be avoided.
In accordance to previous teachings, the coaxial conductors are preformed and affixed in a predetermined pattern on the substrate using an adhesive. A conductive layer forming a ground plane covers the surface of the substrate interconnecting the shields of the coaxial conductors. The intermediate feature includes the step of eliminating the conductive material other than the coaxial signal conductor, around the termination points of the coaxial conductors to provide clearance areas. The clearance then permits connection of the signal conductors to surface terminal pads without shorting to the shield conductors or ground plane.
Another example for providing terminations for coaxial wires is disclosed in U.S. Pat. No. 4,679,321 "Method For Making Coaxial Interconnection Boards" by Plonski. According to the disclosure an interconnection board for high frequency signals is made by wire scribing coaxial conductors. The board is provided with holes. The coaxial conductor with signal conductor surrounded by a dielectric insulator surrounded by a conductive shield are wire scribed on a first surface of the board. The intermediate feature includes the step of stripping the ends of the coaxial conductors to expose signal conductor sections, and dielectric sections. The stripped ends are inserted through the holes such that the dielectric sections reside in the holes and the exposed conductors protrude through the holes on the second surface of the board. The signal conductors are terminated on the second surface, and the conductive shields are connected together on the first surface.
Previous methods of conductor termination, some of which are discussed above, relied on an intermediate feature such as a plated-through hole, a plated blind laser via and plated surface bonding pads to connect the wire conductor to the electronic device. The intermediate features discussed above produce discontinuities in the signal path that may be detrimental to the performance of high speed electronic circuits. Furthermore, on circuits with shielded conductors, the intermediate features represent non-shielded areas which reduce the advantages of the shielding. These features also require more complex processing methods such as drilling and plating for providing through holes from the surface of the board to the signal conductor. The transitions from the signal conductor to plated copper and the surface feature are the areas most likely to fail. Therefore, although insulated wires in general and coaxial conductors in particular are recognized as a preferred interconnect medium where high signal switching speeds are involved, there is a need for an improved termination process to avoid some of the steps involved in providing terminations in accordance with previous methods and systems.